The present invention relates generally to vacuum forming and more particularly to a method for twin-sheet vacuuming forming parts in a vertical orientation.
Vacuum thermoforming generally refers to the process of heating a plastic material in sheet form to its particular processing temperature and forming the hot and flexible material against the contours of a mold by pneumatic means. When held to the shape of the mold and allowed to cool, the plastic retains the shape and detail of the mold.
A particular type of vacuum thermoforming is known as twin-sheet forming. In twin/dual-sheet forming, two sheets of plastic sheet are parallel-laid in a horizontal fashion and heated to a predetermined temperature. A blow pin is inserted between the two sheets and the upper and lower halves of a split mold close onto the sheets and pinch off around the entire perimeter. High pressure is then introduced between the two sheets from the blow pin, and a vacuum is applied to each of the two mold halves. However, a number of limitations exist with current twin-sheet forming technology. Current twin-sheet forming methods require precise controlled heating to ensure quality and consistent output from cycle to cycle. Because the sheets are horizontally oriented, the area in the center of the sheet tends to sag downward from natural gravity. Therefore, very precise and expensive heating controls must be employed to ensure that the plastic is evenly heated. As the material begins to soften it begins to sag downward, drawing away from the top heater and moving closer to the bottom heater, particularly in the center. This action requires precision zone heating controls to prevent the bottom sheet from getting too hot in the center as it moves down toward the bottom heater and the top sheets from being too cool in the center as it moves away from the top heater.
Another cause of sagging is that the melt index of the plastic is too high. Should this be the case, a plastic resin with a lower melt index is required thereby restricting the number of polymers available to utilize in the process.
Another aspect of current twin-sheet forming technology is that of trimming. Because a sheet of material must be clamped on its edges to allow stretching of the sheet into a shape, edge trim must be removed. For low volume, large sized molds, trimming is often manually performed in the forming station while the edge trim is still soft. However, manually cutting the molded article from the sheet is difficult as the sheet is in a horizontal position. This requires the operator to lean over the molded article to reach the outside perimeter of the pinch off. This is often difficult and sometimes dangerous should the operator lose his balance and fall onto the mold itself.
Another problem existing in current twin-sheet forming technology is the clamping pressure required to pinch off the perimeter of the molded article. Because the temperature of horizontally oriented sheets cannot become too hot without sagging, high clamping pressures must be utilized to form an effective pinch off. For high-quality parts, clamping pressures of 150 tons are not uncommon. Exerting this level of force on inexpensive molds often leads to cracking thereby destroying the mold. In addition, high clamping pressure requires more expensive tooling and operation of the apparatus.
Consequently, there is a need in the art for a method of twin-sheet forming that can form large parts without the sheets sagging from the force of gravity.
There is a further need in the art for a method of twin-sheet forming that does not require precise and expensive heating controls to heat the sheet evenly from end to end.
There is a further need in the art for a method of twin-sheet forming that presents the molded part to the manual trimming operator for easy trimming and safe removal of the part.
There is a further need in the art for a method of twin-sheet forming that requires less clamping pressure to pinch off the perimeter of the molded part.
There is a further need in the art for a method of twin-sheet forming that is able to utilize a wide variety of polymers while maintaining a high quality output.
There is a further need in the art for a method of twin-sheet forming that permits the use of less expensive composite molds such as fiberglass, ceramic, epoxy and the like over more expensive molds such as those fabricated from aluminum. It should be noted that aluminum molds may also be used if the part volume justifies the cost on certain models of parts. The present invention can accommodate aluminum and other metal type molds as well. Typically, however, these molds cost more, although usually less than blow molds.
However, in view of the prior art in at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how the identified needs could be fulfilled.
The present invention solves significant problems in the art by providing a method of twin-sheet vacuum forming plastic articles comprising the steps of clamping two parallel sheets of thermoplastic synthetic resin in a vertical orientation to a clamping frame, the two parallel sheets in direct contact with each other about the perimeter of the clamping tray, and blowing air between the parallel sheets thereby creating a hollow cavity between the two parallel sheets. The two parallel sheets are then heated evenly in vertical orientation to molding temperature. The two parallel sheets are disposed between a first opposing mold cavity and a second opposing mold cavity of a vertically oriented split mold wherein the mold cavities have a contour conforming the configuration of the desired article. A negative pressure is applied to suction holes provided in the mold cavities, which draw the two parallel sheets against each opposing mold cavity as the split mold is being clamped together. The split mold is then clamped together to form a pinch off around the perimeter of the hollow cavity thereby forming a molded hollow part still disposed within the clamping frame. Simultaneously, a blow pin inserted into the hollow cavity injects compressed air, further pressing the heated sheets against the mold cavities.
In a preferred embodiment, at least four substantially equidistant spacers are positioned along the circumference of a first mold frame holding the first opposing mold cavity. A second mold frame holding the second opposing mold cavity may be fitted with apertures to slideably receive guide pins projecting from the spacers. The spacers provide a positive stop before the peripheries of the first and second mold frame come into contact. By placing the spacers between the two frames, a gap forms around the perimeter of the first and second mold frames while they are pressed together. This gap provides a pathway for plastic to escape. Otherwise, if the plastic were compressed along the entire perimeter of the first and second mold frames, high press force would be required to press the plastic down in order to properly close the mold. Another benefit of using the spacers is that the pinch offs are protected from damage, particularly when composite materials are used to construct the mold such as fiberglass, ceramic and epoxy. The spacers allow for less press force while providing precise clamping of each opposing mold cavity. Escape holes in the frame under the female guide prevents plastic from building upon the pins.
The molded hollow part is allowed to cool to a set point and the vertically oriented split mold is opened. The mold may be cooled externally with a water spray or may be cooled internally using water conduits.
The molded hollow part may be ejected from the split mold by blowing compressed air through the vacuum suction holes. The clamping tray is withdrawn from the split mold and shuttled away from the mold cavities where the molded hollow part is cut away from the clamping tray, still in a vertical position. In a preferred embodiment, a rail means in provided for sliding the clamping tray to and from the oven, the split mold and a cutting location. The cutting location is adapted to permit manual trimming of the molded hollow part wherein the operator has a physical range of motion free from encumbrances.
An object of this invention is to provide an inexpensive method of twin-sheet vacuum forming large plastic parts in low volumes.
Another object of this invention is to provide a method of twin-sheet vacuum forming that permits higher heating levels.
Another object of this invention is to provide a method of forming large hollow parts without the expense or complexity of blow molding.
Another object of this invention is to provide a method of twin-sheet vacuum forming that produces a high quality article using less expensive plastic resin with a lower melting point.
Another object of this invention is to provide a method of twin-sheet vacuum forming which requires less heat control.
Another object of this invention is to provide a method of twin-sheet vacuum forming that permits easier trimming of the article.
Another object of this invention is to provide a method of twin-sheet vacuum forming large articles without significant deformation from the effects of gravity.
Another object of this invention is to provide a method of twin-sheet vacuum forming that keeps each individual thermoplastic sheet from sticking to each other.
Another object of this invention is to provide a method of twin-sheet vacuum forming that permits the use of inexpensive composite-type molds, such as fiberglass, ceramic, epoxy and the like. Composite materials usually consist of synthetic fibers embedded within a matrix, a material that surrounds and is tightly bound to the fibers. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibers made of a ceramic material such as carbon or glass embedded in a plastic matrix. Typically, the fibers make up about 60 percent of a polymer matrix composite by volume. Metal matrices or ceramic matrices can be substituted for the plastic matrix to provide more specialized composite systems called metal matrix composites and ceramic matrix composites, respectively. These composite materials are generally substantially less expensive to fabricate in comparison to aluminum molds typically used which may increase costs over composites by a magnitude. Traditionally, composite molds have been used in single sided vacuum forming where there are no clamping forces at a pinch off.
Another object of this invention is to provide a method of twin-sheet vacuum forming that requires less clamping pressure and subjects the mold to a reduced amount of press force.
An advantage of the invention is that sheets may be heated to a higher temperature without sagging. Higher heat produces a better pinch off, improved consistency and lessens thinning out of the walls of the resultant part.
Another advantage of the invention is that less expensive polymer resin may be used that has a lower melt point.
Another advantage of the invention is that less heat control is required to maintain a proper shape without sagging.
Another advantage of the invention is that the vertical orientation of the sheets while heating permits the two sheets to remain vertically parallel to both heaters and expand evenly from side to side as they approach molding temperature.
Another advantage of the invention is that the resultant part may be trimmed while standing in a more comfortable and safe position than leaning over a horizontal system.
Another advantage of the invention is that high-quality, large parts may be produced in low volumes without expensive equipment or costly mold methods.
Another advantage of the invention is that it provides manufacturers of low volume, large hollow parts with a less expensive and less complicated means to manufacture parts in-house that were previously molded by using a large blow molding machine.
Another advantage of the invention is that parts can be molded using pre-decorated plastic sheets on one or both sides of the exterior surfaces to create a unique, decorative look such as an automotive rear-deck spoiler so as to give it the appearance that it was made from carbon fiber, wood grain, or other materials. This appearance is impossible to achieve in the conventional blow molding process.
These and other important objects, advantages, and features of the invention will become clear as this description proceeds.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts that will be exemplified in the description set forth hereinafter and the scope of the invention will be indicated in the claims.